Box girder

​When the area to be spanned exceeds that of existing systems then a new system has to be developed. The box girder and the truss bridge evolved together but at different paces depending on the technology available and the geographical location e.g. Europe or USA. Such was the case with the box girder as it evolved from the principle of the initial bridge beam but with the centre hollowed out. Its initial development was led by Robert Stephenson with the box girder so large that the rail went through it. The Britannia Bridge, spanning the Menai Strait joining Anglesey island to mainland Wales, was one of the earliest box girder bridges.

​It was the reconstruction work after World War II that saw the rapid evolution of this type of bridge. New materials, a scarcity of other materials, welding techniques and new design concepts for a box girder produced an economical design where the road surface formed the top of the girder. The first long span bridges of this type were the Cologne – Deutz (1948) with three spans of 132m, 184m and 121m and Dusseldorf – Neuss (1951) with spans of 103m, 206m and 103m.

​Some of these bridges were constructed as cantilevers with the centre piece lifted into position. This type of bridge, while not the most aesthetic to the eye, was very popular and longer spans were being constructed over time until June 1970 when a bridge under construction in Milford on Haven in Wales collapsed, killing four men. Two further collapses necessitated a review of the design forces and the construction techniques and culminated in the Merrison Report. A new British Standard was published to cater for designing bridges of this size.

​The advent of prestressed concrete. where multiple wires or iron bars are tensioned and pass through the concrete, provided an alternative material to steel for the construction of box girder bridges. Whether steel or concrete were used on a bridge depended very much on the availability of the material locally. Concrete bridges were perceived at the time as having lower maintenance costs as they did not have to be painted.

​The box girder deck may be constructed in two ways, namely, precast/prefabricated or in situ. Using precast concrete or prefabricated steel allows for good quality control under factory conditions. The size of the units is usually determined by the size of the largest available lifting equipment and site accessibility. It is usually quicker than in situ construction.

​In situ construction is carried out using a travelling formwork, a temporary metal structure which supports the concrete deck while it is being poured. The size of the formwork depends on the size of the spans but it is usually designed to allow the deck to be constructed in segments while being supported off the previous completed deck. A deck unit is completed on either side of a column so as to ensure balance is maintained, hence it is called balanced cantilever construction. The construction continues to midspan where it then joins the deck construction from the adjoining column. These decks usually incorporate prestressing or post tensioning to facilitate construction and also to reduce the size of the box girder that would otherwise be required.

​The largest prefabricated bridge is the Confederation Bridge in Canada, which joins Prince Edward Island to the Mainland. It consists of 23 spans of 250 metres each. The precast units were 192 metres in length, weighing 8000 tonnes, and the bridge was completed in 1997. The River Rhine in Germany has been spanned by many box girder bridges but a particularly long bridge is the Zoobrücke (Zoo bridge) in Cologne, Germany (1965) with a main steel span of 259 metres.